<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>画圆</title>
	<style>
		*{
			margin:0;
			padding:0;
		}
		body{
			background: #000;
		}
		#c1{
			background: #fff;
		}
	</style>
</head>
<body>
	<canvas width="800" height="600" id="c1">
		<span>请升级版本</span>
	</canvas>
</body>
</html>
<script>
	function d2a(n){
		return n*Math.PI/180;
	}
	var oC=document.getElementById('c1');
	var gd=oC.getContext('2d');
	gd.lineWidth=10;
	gd.strokeStyle='cyan';
	var cx=200;
	var cy=200;
	var r=100;
	function drawArc(x,y,r,start,end){
		start=start-90;
		end=end-90;
		gd.beginPath();
		gd.arc(x,y,r,d2a(start),d2a(end));
		gd.stroke();
	}
	drawArc(cx,cy,r,0,90);
</script>